1. Preheat an oven to 400 degrees F (200 degrees C).

2. Melt butter in a large skillet over low heat. Add onion and cook until soft and translucent, 10 to 15 minutes. Transfer to a 2-quart casserole dish and set aside. Cook bacon in the same skillet over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon on a paper towel-lined plate. Cut bacon into bite-sized pieces and set aside.

3. Combine baked beans, ketchup, Worcestershire sauce, and mustard in the casserole dish. Stir in 2/3 cooked bacon and 1/4 brown sugar until mixture is combined. Top bean mixture with remaining bacon and sprinkle with remaining brown sugar.

4. Bake in the preheated oven until hot and bubbly, about 45 minutes.